The present invention relates to a method for forming counter-sunk seats around holes perforated in a base material and also relates to an apparatus for carrying out the method.
It has hitherto been known to form counter-sunk seats around holes in a base material by means of a metal machining process or a press-working process, and it has also been noticed that the latter or press-working process is more efficient than the former or machining process.
The press-working process has defects in the forming counter-sunk seats. The base material tends to expand radially of the counter-sunk seats as a result of the compression of the base material. The resulting swelling around the counter-sunk seat results in a finished product or merchandise of low or zero utility and strength. Another trouble of the radial expansion is that, in case of forming counter-sunk hole in a long unit of base material which is fed consecutively to a working spot, the dimension of elongation caused by expansion of the material along a feeding direction is added to the original length of the material so as to cause difficulties in forming counter-sunk holes at a correct positions.